Engine detonation control by acoustic methods and apparatus



March 20, 1956 om JR 2,738,781

ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Original Filed July 2, 1951 9 Sheets-Sheet l PRESSURE FQUENC y (/(Moc ya 65) U g' INVENTOR.

March 20, 1956 A. G. BODINE, JR 2,738,781

ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Original Filed July 2, 1951 9 sh t -sh et 2 BY M 0am imui%. f

Jttorngy March 20, 1956 A. G. BODINE, JR 2,73

. ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Original Filed July 2, 1951 9 Sheets-Sheet 3 IN VEN TOR.

Mai M flm m imzdf d w/larngy March 20, 1956 A, G. BODlNE, JR 2,733,781

ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Original Filed July 2, 1951 9 Sheets-Sheet 4 IN VEN TOR.

March 20, 1956 BODlNE, JR 2,738,781

ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Original Filed July 2 195] 9 Sheets-Sheet 5 INVENTOR.

Mai/ 50m v mai or March 20, 1956 A. G. BODINE, JR

ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS 9 Sheets-Sheet 6 Original Filed July 2, 195] a m 1. (M

R. m m a y B March 20, 1956 A. G. BODINE, JR

ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS 9 Sheets-Sheet '7 Original Filed July 2, 1951 By $50M,

March 20, 1956 A A. G. BODINE, JR 2,738,781

ENGINE DETONATION CONTROLBY ACOUSTIC METHODS AND APPARATUS Original Filed July 2', 195] A 9 Sheets-Sheet 8 I 306 IN VENT 0R.

March 20, 1 956 A, BQDlNE, JR 2,738,781

ENGINE DEIONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Original Filed July 2, 195] 9 Sheets-Sheet 9 ENGINE DETONATION CONTROL BY ACOUSTIC METHODS AND APPARATUS Albert G. Bodine, Jr., Van Nuys, Calif.

Original application July 2, 1951, Serial No. 234,688, now

' Patent No. 2,573,536, dated October 2, 1951. Divided and 8this application October 24, 1951, Serial No, 252, 18

8 Claims cl. 123-491 This invention relates generally to internal combustion engines and more particularly to methods of and means for suppressing irregular burning and detonation of fuelair mixture therein. The invention involves the application of certain acoustic techniques. and devices .to combustion engines, and consists broadly in the use of such techniques and devices in combination with combustion chambers of engines. The invention is based upon my discovery that combustion detonation can be alleviated by attacking the problem in the light of theoretical and experimental findings evidencing it to be involved with acoustic phenomena; or by simply applying certain methods and apparatus which can be shown to have certain acoustic features or properties.

The present application is a division of my prior copending application Engine Detonation Control by Acoustic Methods and Apparatus, filed July 2, 1951, now Patent No. 2,573,536.

In the operation of internal combustion engines of the piston and cylinder type, it commonly happens, as a result of improper spark advance, inferior fuels, bad design, improper operation, too high a compression ratio for available fuel, or other reasons, that irregularities of combustion occur during the combustion cycle so that operation of the engine is noisy and its various parts are subjected to sudden violent shocks, and, in some cases, to mechani cal or thermal stresses above those for which they were designed. The term detonation has been applied to a variety of such irregularities in the operation of internal combustion engines, all of which may not result from exactly the same causes or manifest themselves in exactly the same way. Operation under detonation conditions not only results in unsatisfactory engine performance but also in damage to the engine such as enlargement of the bearings, buckling of crank pins, burning and cracking of pistons and rings, buckling of rods, or cracking of the block or cylinder head. Since serious detonation results if the engine is designed with too high a compression ratio for the fuel, one way of avoiding detonation is to design the engine with a compression ratio substantially below' the danger point. Howevetgtit is desirable to use the highest compression ratio possible in order to achieve maximum fuel economy and performance, and most engines, therefore, especially aircraft engines, are designed to operate at a compression ratio which is so high that they are always on the verge of detonating. To'the present time, detonation is controlled by keeping the compression ratio below the danger point, or by use of leaded and expensive ring hydrocarbon fuels. None of these expedients, however, is a fully satisfactoiy solution, as is common knowledge. I l

It is accordingly the primary object of the present invention to provide improved methods and means, based on the science of acoustics, for suppressing or controlling. detonation in internal combustion engines.

While the causes and manner of occurrence of detonation are still subject to research which may reveal new and unexpected aspects, most investigators agree that 2,738,781 Patented Mar. 29, 1956 detonation occurs when normal combustion, at its relatively slowly traveling flame front, somehow causes the pressure and temperature of the last part of the charge to reach its kindling point causing the remaining portion to go off spontaneously and at a very rapid rate; that is, it detonates. The. violent rise in temperature and pressure resulting from this detonation of the last portion of the charge is very often a shock phenomenon which sets up violent compression waves throughout the combustion chamber. I have found that these waves are actually high energy sound waves, consisting of alternate waves or pressure cycles of condensation and rarefaction following one another by 180 in the time cycle, or at least that they include such sound waves to an important extent; and that these sound waves regularly include resonant frequencies causing them to form standing wave patterns in the combustion chamber which may be calculated 1 according to principles governing cavity resonance sound waves. The frequencies of these sound wave patterns are of course modified by the pressure and temperature of the gases involved and the resonant frequencies of adjacent mechanical structures such as the cylinders, pistons, connecting rods, etc., in pressure communication with the combustion chamber gases.

It has been observed that while ordinary normal combustion proceeds with a more or less gradual increase in pressure to a pressure peak, and a gradual decline therefrom, during which any sound waves present are of low order or harmless magnitude, when detonation occurs, pressure builds up with great rapidity to a pressure peak of amplitude substantially in excess of that normally enthis first sudden pressure peaking is followed, usually after amomentary decline, by a secondary and more prolonged phase consisting of succeeding pressure peaks of first increasing and then diminishing magnitude. Careful investigation shows that sound waves of low amplitude and energy content are present in the combustion chamber in the predetonation phase, and that the frequency of the wave pattern tends to increase during the high wave amplitude detonation phases, due very likely to increased gas temperatures caused by the detonation.

My investigations have shown that these phenomena, including the pressure peak or shock phase which often introduces the detonation, are of an acoustic nature developing from one or more points of sound wave origin within the flame in the combustion chamber. The sound waves so generated in the combustion gas travel to and are reflected or echoed by the relatively rigid chamber walls, the successive reflections of waves of resonant frequency probably interfering to re-enforce one another and so promote high amplitude resonant standing wave patcability of the invention in view of its ability to handle acoustic shock phenomena alone.

The extreme stresses set up in certain members of the engine as a result of detonation evidences the occurrence of one or more pressure anti-nodes, but the impact of the violent traveling waves of compression and rarefaction on the reflective surfaces of the engine is enough to account for the usual detonation manifestations. A substantial degree of reflection of the traveling acoustic waves by the walls of the combustion chamber is, I have found, to be expected 3 with the acoustic wave pattern which is inherent in detonation.

The term wave is used generally herein to classify the operation with the usual performance of an extended gaseous body wherein the dimensions of the body are uppreciable relative to an identifying wavelength, which wavelength is calculated from the speed of sound therein divided by the noted pressure cycle frequency. In other words, the term wave is used to denote rapid pressure cycle action in an elastic medium, namely, the combustion chamber gas. This definition applies throughout the specification and the claims.

I have discovered certain intimate connections and coactions between sound Waves and combustion, and these are germane to the invention. In my research on this subject, I have found, for example, that sound waves in a combustion chamber will literally turn a flame off and on, apparently owing to fluctuations in fuel and air density with the compression and rarefaction phases of the sound wave. Again, I have found that this periodic sound-wavecontrolled combustion, because it is a periodic source of pressure pulses, can actually generate or regenerate sound waves. By regeneration, I refer to an action involved wherein an incident sound wave has an effect on combustion activity, and the resulting periodic combustion pressure deviation acts as a thermally driven pressure pulse source to re-enforce the original sound wave, causing a regenerative build-up of both the sound wave and the combustion fluctuation. The combustion chamber of an engine has a tendency to resonate, and when it resonates, there is apparently a secondary extraction of energy from the combustion process which is converted into sonic energy. It is as if a parasite sonic engine process were driven by the engines combustion chamber. The resulting echoing sound wave, reflected back and forth through the combustion chamber, seems to turn the flame off and on, or at least to fluctuate it, apparently as a result of the periodic fluctuation of gas density owing to the sound waves as explained above. This fluctuating combustion is, of course, a periodic pressure pulse source which is ideal for generating sound waves; and of course this sound wave generation, or regeneration, occurs at the frequency of the waves incident on the flame, the wave fre-' quency being a function of echo-time or cavity resonance. The periodic combustion is then a function of chamber resonance which is a function of chamber dimensions and boundary conditions. I have found that this regenerative process, which thus involves a marriage of acoustics and thermodynamics, is the cause of. one of the most troublesome forms of detonation. Apparently an otherwise orderly combustion cycle might divide up into a rapid series of explosions having acoustic features.

The term detonation is sometimes used in a loose sense to cover combustion irregularities of all kinds, even engine knocking due to preignition. It should be understood that my use of the term detonation is confined to the described high pressure cycle phenomena of acous tic causation. With this understanding, detonation in piston forms of combustion engines is found, as already mentioned, in an acoustic pressure cycle pattern having two distinguishable phases, first, a high pressure peak or shock phase, and second, a more continuous or prolonged high amplitude sound wave pattern, of rising and then decaying amplitude. Sometimes the shock phase occurs after a few cycles of high amplitude sound wave pattern, and is then followed by another high amplitude sound wave pattern. The two phases or types, i. e., shock and sustained high amplitude Wave pattern, are usually found together in the acoustic pressure cycle pattern, though it appears possible that either may occur without the other. Both, however, are of acoustic nature.

The high pressure peak or shock wave, which often introduces detonation, is of steep wave front, and when analyzed, reveals the presence of high frequency sound wave components, which are seriously harmful in their action on the chamber walls.

The sustained or continuous wave pattern phase, comgenerative performance continues to a maximum.

monly of rising and then decaying amplitude, occurs, 1 have" found, at one or more resonant frequencies of the combustion chamber. This wave pattern usually, and possibly in all cases, involves a regenerative interaction between the cyclic combustion pressures and concomitant sound waves. The action is apparently somewhat analogous to a phenomenon known in acoustics as the singing flame. The first detonation pressure shock compresses the fuel charge sufliciently that the rate of combustion suddenly increases in a localized region of the chamber; this increases the amplitude of the pressure peak thereat, which in turn launches a pressure pulse which is transmitted with the speed of sound to the combustion chamber walls, whence it is reflected back to increase the combustion rate on the subsequent wave cycle, and so on. The increment of added pressure rise on each half-cycle of the pressure wave is limited by the time interval involved during that half-cycle, of the pressure wave is limited by the time interval involved during that halfcycle, but succeeding pressure wave peaks occuring thousands of times per second are of higher and higher amplitude until a maximum is reached, the limit possibly being imposed by exhaustion of the fuel charge locally by this series of explosions. In other words, each cycle of the sound wave causes the flame to give out a pressure pulse, and each pressure pulse adds acoustic energy to the wave pattern, which cooperative and re- The wave pattern then gradually decreases or decays from this maximum. It must be understood that in the case of piston engines, this high frequency regeneration performance takes place during only a part of the piston stroke. This performance, even if only continued for a few cycles, is very destructive to engines.

Taking into account the acoustic nature or aspect of detonation in combustion, the present invention, in method, contemplates acoustic attenuation of offensive high amplitude sound waves or gas pressure vibrations (including both types discussed above), accompanying detonation in combustion. In apparatus, it comprises an acoustic attenuator, responsive to the high amplitude sound waves or sound wave frequencies of detonation, i. e., to the frequency of frequencies of the acoustic pressure cyclepattern caused by detonation, operatively combined with a combustion chamber, in acoustic communication therewith, and capable of suppressing said waves (or acoustic pressure cycle pattern) sufficiently to render the same harmless. By acoustic communication, I refer broadly to any arrangement by which the detonation sound Waves generated in the combustion chamber can be transmitted effectively between the source of detonation and the attenuator. The communication may be via an open gas path or passage, a vibrating solid medium, through pores, crevices or capillary-like passages, through a body of packed fibers, or the attentuator may actually be within, or form a part of the walls of the combustion chamber, or it may be enclosed inside the chamber. In many cases, the important transmission is from the chamber to the attenuator, such as instances of a non-reflective wave guide and attenuation means which simply absorbs and dissipates the wave without reflection back. In other cases, the more important consideration is the extent to which the attenuator is delivering sound wave energy into the combustion chamber in such phase as to have a cancellation or decoupling efiect on the thermal drive of the sound wave pattern in the chamber. It may also be noted at this point that the acoustic attenuativc provisions used in the practice of the invention are in some cases easily recognized devices or formations, but are often of subtle character, such as acoustically designed attenuative configurations or shapes formed in or on the members defining the combustion chamber, including the top structure of the piston. However, in all cases these are characterized and distinguished from previous combustion chambers and pistons by their acoustic properties.

In the, practice of this, invbntion, acoustic attenuation involves the acoustic suppressionjo'r reduction of the am plitude, intensity, or energy content of these rapid 'pressure variations, th'e' attenuating action being applied in such forms and places asreduetion of geueratiomor of r'eg'eneratiomat the source, orreduction of gasvibration patterns at some point removed from the source. Actu: ally, in the case of standing Twaves,'there is'usually no basic difference where in the wave field the attenuation is applied, because a proper acoustic limitation of a standing wave in one region can affect the wave elsewhere; for example, under some circumstances,spoiling of a wave at a pressure anti-node by'installing a low impedance device there'at can reduce the ability of thesource to drive thewave. Y i Q. i 'r It is not my purpose to suppress all sound waves in the combustion chamber, particularly'since it is apparent to me that a certain degree of controlled sound wave activity, or certainsound wave frequencies, are not harmful, and indeedare actually helpful, perhaps even essential, to an eflicientand rapid combustion process. It is my purpose, however, to control or attenuate the violent, high amplitude sound waves, or sound wave frequencies, which are characteristic of detonation. It a keystone concept of my invention that high amplitude sound waves are not merely an effect or result of detonation, but are part of the detonation process. With this in mind, it is not solely my purpose merely to cushion the engine from pressure wave damage, but rather to attack detonation at its source by selectively suppressing the responsible sound waye pressure cycles. The invention serves either to prevent the effective generation of harmful detonation sound waves in the first instance, or at least to suppress them instantly after initiation, preventing their reflection and re-reflection within the combustion chamber, and also preventing them from reacting regenerativelyqn the'combustion process. I In any event the acoustic basis for deto nation is destroyed.

My acoustic attenuators can be of either or both of two types, viz., energy dissipative, and spoiler.- The first operates by conversion of the acoustic energy of detona tion into heat. The second operates by setting up acoustic conditions incompatible with generation or maintenance of the, offending wave, the latter type being especially applicable to higher modes, which are explained hereinafter. Also, as will later appear, the attenuator often operates partly by dissipation.- action and partly by spoiler action, it being difflcult in some cases to say with assurance which is dominant.

The invention may also be broadly regarded as including the concept of inhibiting or suppressing the resonant characteristics of the combustion chamber, considered as an acoustic chamber or cavity, thus removing or substantially reducing a factor in themaintenance of high amplitude acoustic detonation waves. 'The invention accordingly contemplates the suppression of acoustic Wave reflection and resonance within the combustion chamber as a method and means forv preventing or stifling. the high amplitude pressure wave phenomena of detonation.

Broadly speaking, the present invention provides, in one of its aspects, a combustion chamber which is essentially an -ancchoic acoustic chamber insofar as detonation frequency waves are concerned; By an anechoic combustion chamber I do not refer to' 100% elimination of echoes or reflections, but the very material and substantial reduction thereof, to such an extent that detonation conditions springing from resonance or sound wave reflections inside the chamber are very materially suppressed. Normal combustion may involve certain low amplitude wave patterns, and itis not my purpose to eliminate these, since they are not harmful, and seem likely even to be beneficial to combustion. The anechoic combustion chamber of the present invention th s not da sd' b a s sa ats fitted. hama illustrative embodiments of the invention, in which acoustic attenuator devices are operatively combined with the combustion chamber of an internal combustion engine, and several principal forms will now be explained. in all of these, the acousticattenuator means is designed or selected to be responsive to, that is, to operate at or throughout, the detonation frequencies to be combatted, it being understood that acoustic attenuators in general are not equally responsive to all sound wave frequencies, but, to a greater or lesser degree, depending upon their type, are predominantly responsive to certain frequency ranges. In this connection, it is also found that the frequency response of an attenuator is often a function of its temperature, this following from the effect of temperature on the velocity of sound, especially in the attenuators having gas therein. The attenuator usedis accordingly to be one responsive to detonation frequencies at its operating temperature.

The spoiler type of attenuation will first be considered. The impact of a sound wave in a gaseous body against a relatively rigid reflecting surface is a phenomenon involving high acoustic impedance, by which is understood the ratio of cyclic gas pressure amplitude to gas particle velocity amplitude. Usually a' high impedance region is located at a rigid reflector. The present invention, in the aspect under consideration, contemplates the attenuation of certain sound waves of detonation by interfering with or cancelling, or spoiling their reflection at the relatively rigid high impedance reflecting surfaces of the combustion chamber. This interference is accomplished by introducing, immediately adjacent a high impedance region of the standing Wave, a region of low. acoustic impedance for the frequency of the wave. As implied hereinabove, lowering of the high impedance at one place in a standing wave by installation of an artificial low impedance thereat automatically produces similar effects at all other high impedance regions of the wave. Several types of low impedance attenuators are applicable, including resonators such as Helmholtz resonators and quarter-wave cavities or spoilers. The Waves of standing wave frequency generated by the detonation thus travel from their points of origin to the reflecting surfaces or. high impedance regions of the wave, but there meet two incompatible high and low impedance conditions, which substantially spoil the reflection, thus preventing the regenerative build-up of this standing wave system by the source. In effect, the low impedance region short circuits the high impedance region; and, of course, the combustion wave-source cannot build up the wave in the presence of such a critical short in the acoustic circuit. The detonation waves are thus substantially attenuated back at their point of origin Within the flame by attenuation action imposed at the surfaces of the chamber. Accordingly, the phenomena of wave reflection with regeneration and large amplitude wave. patterns are substantially eliminated for the chosen 'wave frequency, and no large stresses therefrom are mam , 7 is useful with the regenerative type of thermal source which depends upon a coacting wave, and particularly also with a source whose operation depends upon its being located at a high impedance region of a wave pattern. I

The same Helmholtz or quarter-wave resonators are also dissipative of acoustic energy, especially for high amplitude acoustic waves such as are involved in the problem at hand. The dissipation comes about through flow losses owing to such effects as gas turbulence, heat conduction to the walls of the resonator, and thence to the cooling system of the engine, and rectification of pulsating acoustic energy, into direct current flow energy. So long as the resonators are designed in accordance with the acoustic principles, as taught herein, it is not too im portant whether they attenuate primarily by spoiler action or primarily by dissipation.

In a typical and illustrative practice of the invention, the walls defining the high impedance reflective surfaces of the combustion chamber are provided with or breached by a plurality of acoustic wave attenuation cavities in the nature of resonators, which communicate through said surfaces with the combustion chamber. In other words, they are in acoustic communication with the combustion chamber. These cavities (which may be used in the walls of the cylinder head, or in the end of the piston, or all of them) are precisely designed or tuned to be resonant to the predominating detonation frequency, which is very often of the order of 5,000 to 10,000

C. P. 5. They may typically consist of Helmholtz res onators, or quarter-wave pipe resonator cavities or spoilers.

Assuming quarter-wave pipe resonator cavities in the form of cylindrical drilled holes, a detonation frequency of 9,000 C. P. S. to be suppressed, and sound wave velocity of 3,000 ft. per second in the heated gases, it is apparent from elementary sound wave theory that the depth of the cavities will be substantially one inch. (Frequency times wavelength equals wave velocity.) The operation of this specific type of cavity may be explained in terms of the sound wave reflection that is characteristic of quarter-wave pipes. Thus a wave of condensation (positive pressure wave) approaching the high impedance reflecting surface surrounding or adjacent the cavity will tend to be reflected by the surface, and a portion will also tend to enter and traverse the wave guide formed by the cavity. Reflection will occur at the inner end of the cavity, and, assuming proper cavity depth, the wave will return to the mouth of the cavity as a reflected wave of condensation just 130 following the instant of its initial entry. But at this time the wave of rarefaction (negative pressure wave) that follows the original wave of condensation by a time lag of 180 has occurred in the main chamber in the region of the reflecting surface and the mouth of the cavity, and this wave of rarefaction and the reflected wave of condensation emitted from the cavity neutralize and cancel one another. Thus the original wave train is split into two mutually interfering components of substantially 180 phase difference which act to destroy one another, and so eliminate the effect of the wave train, including its effect upon the flame. It should be appreciated that just any depression or cavity will not suffice; an actual practice of acoustics, assuring the necessary phase relationships and/or relative acoustic impedances is necessary.

The analysis in the case of Helmholtz resonators must be somewhat altered in view of the absence of the simple phenomena of reflection time from a closed inner end of such a resonator. The Helmholtz resonator will nevertheless act to return a received pressure pulse in like kind (positive or negative) 180 in time cycle after its reception, and it hence behaves in the invention as the equivalent of a quarter-wave cavity. The use of both quarter-wave cavities and Helmholtz resonators may however be explained'in terms of acoustic impedance, both operating at the accurately determined frequency to present a low acoustic impedance to the unwanted sound wave mode at a region adjacent to a high impedance refleeting region for the wave, and thus serving to short circuit the high impedance reflecting region with the result of spoiling the wave.

The invention has now been described in a form employing Helmholtz and quarter-wave resonators, both of which are effective in either or both of two ways, viz., as spoilers, and as dissipators.

As alreadycxplained, a Helmholtz .resonator is not only able to limit a sound wave by providing a region of low acoustic impedance adjacent to a region of high impedance, thus reducing wave reflection, but also by dissipating acoustic energy. If thecross-sectional area of the neck of the resonator is ample, large amplitude waves at its resonant frequency will be attenuated or absorbed primarily by flow losses in the region of the neck of the resonator caused by a mass plug of gas oscillating back and forth therein with large velocity against the spring resistance offered by the gas volume in the cavity of the resonator. The Helmholtz resonator suggested above may also be used in a further and different manner. With a neck of greatly reduced cross-sectional area, e. g., near capillary-like dimensions, the waves may be absorbed by viscosity. Attenuation by flow effects and viscosity both come under the general heading of acoustic dissipation. In a modified form of the invention, I employ one or more Helmholtz resonators formed or installed in or near the walls of an internal combustion engine, together with a means in the neck of each such resonator for giving the passageway a high resistance or dissipation factor owing to viscosity. This result is accomplished by placing in the neck of the resonator a porous barrier in the form of a screen or porous or iibrous plug, wall or body, presenting to the incident waves an array of tortuous passages of small transverse dimensions, some of which may terminate within the body of the barrier, and some of which may extend entirely therethrough. It must be understood that a porous barrier or body will cut down the cross-sectional area of the neck and thus lower the frequency of the resonator so that it may have to be tuned as an assembly. The devious passages with which this body is honeycombed offer very great resistance to a broad frequency band of waves owing to the effect of viscosity which comes into play in the high velocity, low impedance neck of the resonator when the passages are sufficiently small in transverse dimension.

Used within the neck of the resonator, the main benefit of such a porous body is to materially reduce the Q of the resonator, flattening the peak of the resonance curve, and therefore increasing the frequency band width which will be subject to attenuation by the resonator. The resonator will still have the necessary characteristic frequency response with a resonance peak for waves of a certain frequency, but the peak will not be so sharp. In practice, the resonator is designed to have its resonance peak near the principal detonation frequency, and by virtue of the broadening of the wave band resulting from use of the porous body, not only that frequency, but other frequencies for a considerable range on either side thereof are materially attenuated. In this connection, it should be understood that the true objective is that the attenuator be "responsive to, i. e., have a material attenuative effect on, the offensive detonation frequency, and that I employ the expression frequency response to denote an attenuative response or action for a frequency which is to be suppressed. In some cases the device may desirably have a relatively high Q, but in others, e. g.,

Where a single attenuator is to suppress two or more separated detonation frequencies, or a relatively wide band resonant peak, broadness of tuning is desirable. Thus a broad tuning device, such as the form of attenu- 1'1 block and head of a modified form of'engine incorporating a further modified form of the invention;

Figure 27 is a detail of a modified combustion cham-' ber extension capable of substitution for that shown in Figure 26;

Figures 28 to 30 are longitudinal sectional views through auxiliary devices embodying different conversions of one form of the invention, and capable of typical use with the engine of Figure 21; v

Figure 31 is a vertical transverse section through the block and head of a modified engine incorporating another form of the invention;

Figure 32 is a developed view of the horn of Figure 31, taken in accordance with a curved plane passing through the line 3232 of Figure 31;

Figure 33 is a bottom plan view of the horn member of Figures 31 and 32, showing the curvature of said member as seen in a horizontal plane; I

Figure 34 is a plan view of a piston incorporating a horn type of wave guide and attenuator;

Figure 35 is a section taken along the line 35-35 of Figure 34;

Figure 36 is a plan view of a piston incorporating another form of horn type wave guide and attenuator;

Figure 37 is a section taken on line 37--37 of Figure 36;

Figure 38 is a view similar to Figure 36, but showing a modification;

Figure 39 is a fragmentary sectional view through a piston showing a modified form of horn type Wave guide and attenuator;

Figure 40 is a partly elevational, partly sectional view of a piston configured to provide a horn type wave guide and attenuator between its periphery and the wall surfaces of the cylinder.

Reference is directed first to Figure 8a, showing a typical or illustrative pressure cycle pattern occurring in a detonating engine combustion chamber as viewed on a fast sweep oscilloscope driven by a sufliciently sensitive pickup connected to the chamber, The pattern shows the two previously described phases of detonation, the initial shock phase, indicated at A, and the prolonged secondary phase, of rising and then falling amplitude, at B. The pressure cycle pattern varies with ditferent engines, different fuels, and different conditions of engine operation. Sometimes the initial shock phase does not appear, sometimes it may suddenly occur after a few cycles of rising pressure amplitude, and sometimes the detonation may be almost entirely of shock wave character. Again the shock wave may intervene in the course of a prolonged high amplitude pressure wave performance. Figure 8a is accordingly merely illustrative of one typical pressure cycle pattern under detonation conditions, showing in thisinstance the two characteristic types of pressure wave manifestation found in detonation. My investigations have shown that this detonation pressure cycle pattern, in both its described phases, is of acoustic nature, and that at least the prolonged secondary phase, as described above, occurs at resonant frequencies (fundamental and higher modes) of the combustion chamber.

The modern trend in engine design is toward valve-inhead engines of simple, symmetrical combustion cham ber shapes, typically geometric figures of revolution. in line with this trend, i. have chosen, for some of my test work, a valve-in-head engine having a flat pan-cake cylindrical combustion chamber such as is used in the well known Cooperative Committee Test Engine known as the C. F. R. engine. With this engine I have made a thorough acoustic analysis of the combustion chamher when the engine is not running; and using this analysis, I have been able'to make the combustion chamber substantially anechoic for detonation wave patterns. Thereafter the engine has operated substantially free of 12 detonation at compression ratios ranging between 8 to l and 12 to 1, using. low grade fuel. Discussion of this treatment of a typical engine will greatly aid the understanding of the invention. I

This test engine had a 3 inch bore, and the piston was usually between k and /8 inch distance from the head when detonation occurred. I drilled an opening through the cylinder head and installed therein an acoustic driver consisting of an oscillator driven transducer having a relatively flat output characteristic for the range of frequencies of interest. This transducer, consisting of a stack of magnetostrictive laminations, was driven by an adjustable frequency electronic oscillator so that the transducer could be made to generate sound waves of any desired frequency in the combustion chamber. For exploring the acoustical patterns within the combustion chamber, l placed a piston in the bore and adjusted its distance from the cylinder head to a distance of approximately inch. I drilled a small opening through the piston head, centered 1 /3 inches from the center of the piston, and mounted in this opening a high fidelity condenser microphone having a fiat response curve. Then, by rotation of the piston in the cylinder, the acoustic standing wave at different locations around the combustion chamber could be explored.

The piston was rotated to a position wherein the microphone carried thereby picked up a strong signal. Then, hyvarying the frequency of the driver from a few hundred cycles per second to about six thousand cycles per second, I obtained the pressure amplitude response curve shown in Figure 5. When the driver frequency passed through a resonant frequency of the combustion chamber. the amplitude, as registered by the pickup microphone, was very high. It will be noted from Figure 5 that the cavity formed by the combustion chamber had a very high Q; that is, it tuned sharply, giving a very high amplitude in a very narrow frequency band. The radial mode (second overtone), occurring at approximately 5,000 cycles, was very weak compared with the fundamental, because neither the microphone nor the transducer were at the center of the cylinder where the radial mode has its optimum high impedance region. However, as shown by the graph of Figure 5, it was nevertheless possible to determine the frequency of this radial mode.

To explore the standing waves in the combustion chamber, the piston, with the microphone mounted therein as heretofore described, was rotated so that the microphone area swept the standing wave pattern. To simplify the analysis for each resonant frequency, the driver was adjusted for the purpose of each such exploration to the single fixed frequency for each of the resonant peaks shown in Figure 5. By this procedure, it was determined that certain characteristic standing wave patterns such as shown in Figures 1 to 4 were present in the combustion chamber, the four diagrams of Figures 1 to 4 being of the standing sound wave patterns corresponding respectively to the first four resonant peaks up to 6,000 cycles in the graph of Figure 5, including the small one at 5,000 cycles.

The standing wave pattern for each mode (resonant peak) was determined by counting the number of pressure anti-node regions P (high impedance regions) where the microphone gave maximum reading. The corresponding velocity patterns as shown by the full line and dotted line arrows was then postulated from known facts about cavity resonance. In these diagrams, the full line arrows represent the gas particle velocity for one phase of the acoustic standing wave pattern, and the dotted line arrows represent gas particle velocity for the succeeding phase. That is to say, for of duration of each cycle of the ,standing Wave, the gas particle velocity is in the direction of the full line arrows, and for the succeeding 180 the gas particle velocity is in the direction of the dotted line alternately from one of these to the other, andthen in,

thereverse direction. This is sometimes known as the sloshing mode. Figure 2, representing the first higher mode (cold air frequency of approximately 4,000 C. P. 8.), shows that there are four high impedance pressure anti-node sectors P, with gas flow regions therebetween having alternate flow patterns as represented by the arrows. Figure 3, showing the second higher mode (cold air frequency of approximately 5,000 C. P. 8.), it reveals that this is a radial mode. It was not possible to fully explore this mode with the microphone located as described, and the weak response shown in Figure is owing to the fact that the microphone could not be positioned atthe high impedance pressure anti-node regions. However, it was possible to make out the pattern, which involved a high impedance pressure anti-node region P at the center, a smgle continuous circumferential high impedance or pressure anti-node region P around the periphery, and radial velocity flow patterns as indicated by the arrows. Figure 4 shows the third higher mode (cold air frequency of approximately 5,700 C. P. 8.),

whose pattern is mode, excepting V with intervening velocity anti-node regions.

essentially similar to the second higher "The actual angular location of the pressure anti-node;

and velocity regions depends upon the location of the driver. The driver locates one of the high impedance regions (pressure anti-node), and all the other .regiousof,

the" pattern then locate themselvesaccording to the laws of acoustics; in the case of a circular, combustion chamber,

the distribution of the pattern is equiangular, as represented iii Figures 1-4. The location of the driver controls the orientation of the pattern, but the equiangular relationship between pressure and velocity anti-nodes is unaffected by driver location," With unsymmetrical com bustion chambers, such as in the patterns would of course With symmetrical chambers it can be depended upon that whatever the location of the driver, which in an, actual engine under running conditions is a source, point in a flame of not easily predictable location, acoustic patterns such as shown in Figures l-4,will be established, though their orientations about the axis of the combustion chamber will not easily be known. Furthermore, inthe actual engine, several parts of the flame may function as separate drivers, and a corresponding plurality of similar acoustic patterns may then be superimposed one over another, but with no necessary correlation of orientation between the patterns.

Accordingly, while the acoustic standing wave patterns are'ascertainable, including the spacings of the high impedance regions, it is not easy to determine the actuallocations of the high impedance regions, where attenuators might be installed to maximum effect. The invention meets the problem of attenuator location in difierent ways. First, and this is particularly applicable to unsymmetrical chambers, where no symmetrical acoustic pattern of pressure anti-nodes 'could be anticipated, I employa substantial number of individual attenuators distributed throughout the combustion chamber, andthese may be located in the upper part of the cylinder wall and in the cylinder head, or in the piston, or both. In another embodiment, I employ a single horn-shaped extension of L-head engines, most of not be symmetrical.

the combustion chamber, typically connected into thev cylinder head directly over the piston, together with an. attenuator at the throat of this horn.

In one embodiment for symmetrical chambers, I employ just a few attenuators, but so located and spaced 'for having siX pressure anti-nodes P,

from one another as to assure attenuation of the acoustic wave pattern regardless of whether or not the attenuators" coincide precisely with the high impedance pressure antinodes. Looking at any one of Figures 1 to 4, it can easily be seen that; the low impedance veloQit-yanti-node, regions (,identifie'd by the locations of the arrows), lie midway.

between the high impedance regions Pl Therefore, two attenuators can be installed with a spacing approximately equal to half the spacingbetween two high impedance regions. Thenif one attenuator, happens to. be too near a low impedance region for substantial effectiveness, the other will automatically be. suiliciently near ahigh impedance region for, substantial effectiveness. Such spacing canbe called quarter-wave spacing because of the analogy to parallel beam transmission where the distance between high and low impedance nodes is'equal to one-quarter wavelength measured along the parallel beam. in this instance an angular division'is the simplest. In one later described embodiment, I employ three horn type attenuators mounted in the piston" head, positioned near the periphery thereof, and angularly spaced 4 5 apart. The two outside attenuators are therefore spaced apart, taking care of the acoustic pattern of Figure 1. That is, if one happens to be in the region of the arrows, the

other will be near a P region. Any two adjacent attenuators are 45 apart, taking care of the mode of Figure 2. Apparently the three attenuators at 45 spacing average out well enough to substantially attenuate the mode of Figure 4, although the'lideal spacing for this mode' would be 30. rim, they then also act onthe radial mode shown in Figure 3. Figure 6 is a graph showing the resonant characteris'ticsof the combustion chamber equipped with the three spaced horn-type attenuators as here described. Itgwill be seen that the high resonant peaks offFigurefi, obtaining with the chamberin its natural resonant state, have been lowered to a small fraction of their/initial heights. In subsequent'actual running tests, the engine with the attenuators installed as just above described, and with. I

apparently knock-free" as indicated by listening and by using conventional detonation pickup equipment 7 The graph of Figure 7'shows the effect of using a quarter-wave 'pipe resonator cavity, I a too high Q. The device tuned sosharply that, it tookout only the center portion of the resonance peak of 'the first mode, leaving twin peaks of considerably reduced amplitude, but still too high for complete satisfaction.

The graph of Figure 8 shows the result of using a small diameter quarter-wave absorber having lower Q, Very probably the dissipative ability of this low Q absorber can explain a substantial portion of its attenuating action, separate from the spoiler hypothesis. This attenuator was effective in reaching out across the entire first mode resonant hump, although it was too sharply tuned to reach other resonant peaks. It may be observed that, with this characteristic, it. is possible to employ resonant absorbers responsive to'the band width of a given resonant hump but without effect on wave, frequencies between humps. This type of attenuator, is accordingly often 7 preferred when it is, desired to remove the resonant peaks, but topermit free. play of waves of small amplitude in thefrequency brackets between resonant peaks. l

in which:

fis frequency c is speed of sound d is diameter of bore If they are alllocatednear the l acoustic combustion chamber char acteristics as represented by the graph of Figure 6, ran

in 'this instance, with.

x is wavelength k is the Bessel function parameter defining each radial or circumferential mode.

I have found further that for the modes of Figures 14, the following table, derived from Bessel functions, can be used:

Figure: k 1 0.59 2 0.97 3 1.22 4 1.34

Substituting in the above equation for will give frequencies agreeing with the above-mentioned experimentally determined frequencies when we assume c (speed of sound) 12,480 inches per second in a cold metal chamber, and d=3 inches (3 inch bore). The values of k for each mode are taken from the table. Frequency increases at high temperatures because c (speed of sound) increases. However, this is not too bothersome in most of my designs for attenuators because they usually are designed in accordance with standing wavelength which is not variable with temperature, as indicated by the formula for above.

In Figure 9 is shown an L-head engine comprised of a water cooled block 10, a water cooled head 11 fastened to block by means of studs 12 and nuts 13, a piston 14 reciprocable in cylinder 15 in block 10, an exhaust valve 16, and a spark plug 17 approximately aligned with the valve 16. It will be understood that, as in conventional L-head engines, an intake valve (not shown) will be located alongside exhaust valve 16, such valve being of course out of the plane of the drawing. Block 10 and head 11 have cooling jackets 18 and 19, respectively, and head 11 has an inner combustion chamber wall 20, braced by webs 20a, which encloses a combustion chambcr space 21 over the cylinder and valve, as shown. The combustion chamber walls, including the upper end of the piston, the upper end portion of the cylinder, and the wall 20, are formed with or breached by a substantial number of acoustic spoiler cavities or resonant absorbers, here in the nature of pipe resonators consisting of straight cylindrical bores 22 opening inside the combustion chamber. The webs 20a furnish stock in which some of the bores 22 can be formed, and are designed with a spacingdistance for proper spacing of the bores 22. It will be seen that regions of the combustion chamber wall surfaces through which these cavities 22 open into the combustion chamber are the high impedance reflecting surfaces of the combustion chamber near high impedance regions of a wave pattern; the cavities 22 then provide low impedance regions interspersed with or located within these high impedance regions of the wall surfaces. While these spoiler cavities or resonant absorbers 22 may be of various shapes and cross sections, they are shown in this case as being substantially cylindrical and straight, such as may conventionally be formed by drilling. In some cases I find it desirable to have a chamber or radius on the edge of the hole.

As previously explained, the cavities 22 as shown in Figure 9 are designed to-function as quarter-wavelength pipe resonators at the detonation frequency. Assume for example that a given engine has been determined, by test, to be subject to detonation at a frequency of about 9,000

C. l. S. and that it is further determined that the'velocity.

of sound is 3,000 feet per second in the heated combustion gases within the combustion chamber, it is apparent that the wave length of thedetonation wave is 4 inches. In such a case, the cavity depth for resonant tuning would be very nearly 1 inch, taking into account the well known end correction for quarter-wave pipes in other usages. Assuming that detonation has been foundto occur also at some additional frequency, or frequencies, as is usually the case, some of the cavities are preferably designed to ymnast cavities.

have depths similarly calculated to approximate quarterwavelengths for any such further frequencies. Still further, in view of the fact that in some engines, i. e., those tending to pump oil, the cavities may tend to gradually accumulate carbon at their inner ends, some of the cavities are in some cases designed to have initially a depth a little greater than one-quarter Wavelength for the detonation frequency to which they are designed to respond. Such over-length cavities may not initially be effective to the maximum degree, but as cavities of precisely determined quarter-wavelengths tend to accumulate carbon, and hence to decline in effectiveness, these initially overlength cavities will accumulate carbon until their lengths are exactly quarter-wave, and thus acquire maximum effectiveness. The cavities have, of course, been so designed that carbon deposits can readily be removed at any time that the cylinder head is off.

The depths of the spoiler cavities for resonant tuning being thus determined, it is next necessary to determine their optimum cross section, and also their spacing from oneanother for optimum results. The cross section of the-cavities relative to their depths is one of the factors which govern the Q, or sharpness of their tuning, i. e., the narrowncss of the band of sound wave frequencies to which they will respond and therefore which can be absorbed or suppressed. It is desirable that the sharpness of tuning be not too pronounced, lest the band width removed become narrower than the band of offensive detonation frequencies. Particularly for dealing with the fundamental frequency of the chamber, I find it desirable to include a substantial dissipative factor for actual wave absorption, and this lowers the Q of the spoiler. It is also important, on the other hand, that the cross section of the cavities, or their number, he not too great, since it always remains desirable to retain a preponderance of high impedance reflective area relative to low impedance cavity area. Apparently this precaution assures large oscillating flow rate in the dissipative mouths of the This last-mentioned condition is satisfied when the area of the cavity openings is made to be less than one-half the entire combustion chamber area above the piston at the'time of explosion.

With respect to the relation between cavity depth and cross section, I find it desirable that the diameter of cylindrically formed cavities preferably not exceed substantially one-half their depth, and I have found that a diameter of about'one-tenth the depth gives good results and affords a substantially dissipative factor. The spacing of the cavities from one another is also a factor of importance. In general, I have found it to be desirable to have this spacing somewhat ample, i. e., approximately aquarter-wave length. By making the spacing substantially a quarter-wave length, lateral waves tending. to travel along the combustion chamber wall surfaces are attenuated, since such a wave permitted by one of a pair of quarter-wave spaced cavities will be attenuated by the other. This selfcorrected spacing idea was explained with reference to Figures 1-4 above.

In the operation of the engine of Figure 9, sound waves developed within the flame upon the onset of detonation are materially attenuated at the combustion chamber walls, inhibiting thermal-wave coaction in the manner already described in the introductory paragraphs. Briefly, a sustained sound wave of compression or condensation emanating from a detonation origin point within the flame approaches the several combustion chamber walls, and parts of the wavefront impinge on the high impedance reflecting walls, while other parts enter the low impedance cavities. .The last-mentioned parts of the wave are rellected and returned to the mouths of the cavities as positive pressure waves after their entrance, and therefore coincidentally with the arrival of the wave of rarefaction which follows the original wave of condensation by a time lag of 180. The waves of condensation returned from the cavities tend to cancel the arriving wave as well.

if rar a hta d awe sna t as t fisbe (ve y materially suppressed by cancellation. Perfect 1100% cancellation is of course not to be expected, but the common "detonation manifestations can be virtuallyeliminated; I

WAS previously mentionedpruse of the" cavities 2'2 neutralizes a certain band of detonation frequencies, 'neutralizationbeing most complete for the frequencytat the middle of the band, and tapering offin'both directions. I The width of, such a,ba nd and thelfsharpness oftuning area of the combustion chamber, each cavity having relatively sharp tuning (high Q),and the diiferentdepths being selected so as to assure coverage of the full detonation frequency band. V

In addition, itcan be said that the resonator cavities 22 attenuate the detonation sound waves by dissipation of the acoustic energy, which is converted into heat asa result of flow factor losses, as described earlier. I have also discovered, as already described, that detonation in a combustion chamber may involve more than one detonation frequency band. In such case, of course, use may be made of two or more sets of intermixed cavities having depths properly related to the twoor more detonation frequency bands for neutralization or dissipation of said bands. When cavities designed for more than one frequency are being used in the combustion chamber, it is possible to'increase the total number of cavities per unit area because anattenuator cavity tuned to a given frequency, and presenting a low acoustic impedance for that frequency, will nevertheless present a" relatively high acoustic impedance for other frequencies provided, of course, they are not exact multiple harmonics or overtones. Inother words, cavities designed for a given frequency, or frequency band, do not greatly reduce the high impedance reflection area of the chamber for the other usual frequenciesj It should alsofbe recognized that quarter wav e cavities of the type of Figure 9 present low acoustic impedance "not only forithe fundamental frequency, but for odd harmonics of that frequency, and

hence are attenuativenot only for the fundamental frequency forwhich they are designed, but for harmonics Figure 10 is a fragmentary sectional view of a modified combustion chamber wall 2021 showing the use of Helmholtz resonator cavitiesi22 awhich are characterized by lumped constants rather than the distributed constants of the quarter-wavelength cavities 22 of Figure 9. As is well known in" the acoustic art, Helmholtz c avities such as indicated at 220 are resonant to a given frequency, and provide a low impedance for that frequency. The manner in which they do this, however, is better explained without the analogy ofthe reflections described in con- .nection with the quarter-wave cavities 22 of Figure 9.

Their ultimate effect the same as that of the quarterwave cavities 22, since they present a low impedan'cefor their resonant frequency, and since they will return a positive pressure pulse to their mouths 180 of time lagjafter entrance. Cavities of the type of Fig'ure 1 couldhence be substituted for those" of Figure 9 without alteringthe behavior of the system. Both pipe resonator and'Helmholtz resonator cavities are known in the art of acoustics as resonant absorbers, i'n ViCW oftheir ability to dissipate a sound wave of frequency to which they areresonant. Both the pipe resonator cavities of Figure 9 and the Helmholtziresonator cavities of Figure have also this dissipative property, particularly for the high amplitude waves caused by detonation, and may be broadly I mam quarter-wave pipe resonator type of resonant absorber, such as discussed in connection with Figure 9, but are provided with angular intermediate sections 26, so that the inner end portions of the cavitiesare laterally offset from theentrance ends thereof; By this arrangement, the interiors of the cavities are somewhat shielded from the heat produced within the combustion chamber. In this connection, it is found in practice that carbon tends to accumulate on surfaces 'which are in a certain intermediate heat range,and that surfac'es maintained at either a relatively high temperature, or at a relativelylow temperature, will not accumulate carbon. This is believed to be due to the fact that certain varnishesefe produced in the combustionproeess. These varnishes are capable of adhering to combustion chamber wall surfaces within a certain heat range, so providing, conditions favorable to an accumulation of carbon. Both above and below such heat range, the varnishes do not appear on the combustion chamber surfaces and the carbon does not deposit. The embodiment of Figure 11 will be seen to be one means providing offset shielding from radiation for keeping the inner end portions of the resonant cavities at an operating temperature below thatfor which carbon will deposit. The heat radiating fins 25 shown on the combustion chamber wall 20b are of course also for the purpose of keeping the combustion chamber wall and its passageways 22b at a cool operating temperature;

Referring again to the cavities 22in the top of the piston of Figure 9, I have found that the piston is an advantageous location for the attenuator, which operates very well even though the piston is moving, I have shown herein a number of piston carried attenuators in addition to the embodiment of Figure 9, some consisting of auxiliary devices secured to the piston structure, and some, such as shown in Figure 9, and also in Figures 34 to 38, inclusive, and Figure 40, embodied or formed in the structure of the piston itself.

Figures 12 and 13 show a modification, wherein acoustic attenuators in;th'e form of half-wave" pipe resonators 27 are'mountedin the piston top 28 (which of course constitutes one wall of the combustioii chamber). As' shown, the piston top 28 has cast therein a U-shaped tube 27, whose two open ends open through the top surface of the piston. The length'of this U-tube is one-half the wavelength or the detonation sound wave to be combatted. ln effect, within its frequency range, the U-tube functions for the purpose herein as two individual quarter-wave pipes and the'terrn quarterwave pipe or cavity in'the subtended claims includes these double-pipe, or half-wave forms. Thus a positive pressure wave of tuned frequency generated by a detonation origin point in the flame and incident upon either or both of the two ends of the U-tub'e will cause it to resonate and thereby function as a resonant absorber. This double, or continuous passage type, is relatively free of carbon accumulation problems.

Figures 14' and 15 show another piston mounted pipe type of device which I prefer" to call a wave canceller, designed for combatting first higher mode acoustic patterns as shown in Figure 2. In this instance, there is cast in the head of piston 29 a tube 30 having its two ends opening through the top of the piston at spacing from one another, the openings being here shown as located about three-quarters of the way from the center of the piston to its circumference, and at two of the high impedance pressure anti-node regions P. Such location 'is made following appropriate probing of the wave pattern. The tube 30 here has a length equal to one full wavelength of the offensive detonation frequency,

19 always in phase. .Now, a study of the wave pattern in the cylinder over the piston (Figure 2) shows it is a first higher mode, and the pressure peaks at the 90 spaced pressure anti-node zones marked P in Figure 14 hence occur with 180. phase difference. This means that a positive pressure pulse transmitted through the tube from one end thereof to the other will arrive at the far end of the tube coincidently with the occurrence of a negative pressure pulse of the first higher mode wave pattern at the region of saidfar end of the tube, and the result is a cancellation or attenuation of this mode. This first higher mode is chosen as a convenient example; however, the procedure explained therefor can be applied to other modes, it being only necessary to pipe out-of-phase sound to a high impedance point. For

taking care of other possible orientations of the pattern, another pipe shifted in location 45 will do, as explained above.

Figures 16 and 17 show a modified piston 31 whose top is formed with cylindrical sockets 32 to receive Helmholtz resonators 33. These resonators each have a cylindrical body 34, rounded at the bottom, and formed at the upper end with an external annular flange 340 which snugly fits the socket 32, the body 34 otherwise remaining spaced from the piston structure excepting at its lower end. Heat conduction to the piston structure is thus reduced, and the resonator operates at a high temperature. At the bottom, body 34 is provided with a stern 34b extending down through the head of the piston, its lower end being riveted, as at 340, to secure the resonator tightly in position. The resonator is provided with atop wall 34a, furnished with a downwardly extending tubular stem 34e which opens into the lower portion of body 34, this stem 34:? being the neck of the resonator. Y The farthest inner end portion of the cavity of this resonator, where any carbon might tend to accumulate, will be seen to be the top which is turned toward the heat to give a condition of temperature substantially above the aforementioned range where carbon will deposit, with the result that carbon accumulation within the resonator is minimized or prevented.

Reference to Figure 16 willshow that a plurality of resonators has been mounted in the top of the piston in a pattern designed to attenuate the fundamental and also the various higer modes indicated in Figures 1 to 4. Thus, two of the resonators 33 have been mounted at 90 spacing from one another, and these are effective on the fundamental. Such a pair of resonators can be installed, knowing that the type of wave pattern of Figure 1 exists, but without knowing the precise location of the pressure anti-node regions. If one attenuator happens to be too near a low impedance region for substantial effectiveness, the other will automatically be sufficiently near a high impedance region for substantial effectiveness. Then, to combat the first higher mode, two smaller attenuators 33a, which may be like the resonators 33, except to be more attenuative of the first higher mode frequency, are mounted in the piston top at 45 spacing. For the purpose of the third higher mode, two still smaller resonators 33!) are mounted in the piston top at spacing. A single resonator 33c is mounted in the exact center of the piston to combat the second higher mode, which is known to be a radial mode and to have a pressure anti-node over the center of the piston. The resonators33 to 33 are of course designed in each instance to have proper dimensions to be resonant for the respective fundamental and higher mode frequency which they are to attenuate. The dis tribution of attenuators as shown in Figure 16 assures location of at least one attentuator sufliciently near to a high impedance region of each of the illustrative wave patterns shown in Figures 1 to 4.

Figure 18 shows another modification of piston mounted Helmholtz resonator, the Helmholtz 'resonator being formed in this instance by a bore 35 extending upwardly v 20 through the top 35a of the piston nearly to the top surface thereof, and by a smaller bore 35b extending through the top surface of the piston, the latter forming the neck of the resonator. The bottom of the bore 35 is then closed by a plug 350, which may be secured in place by peening as at 35e. The Helmholtz resonator in this form is of advantage in view of itssimple nature and the ease with which it may he formed in a conventional piston structure. i

Figure 19 shows still another piston mounted Helmholtz resonator type of absorber. Here, the piston top 36 is formed with a bore 36a extending downwardly from its top surface, and an enlarged counterbore 36b extending' upwardly from below. The Helmholtz resonator structure, indicated generally by numeral 37, includes a cylindrical base portion 37a, received in counterbore 36b, and secured in position by peening as at 37b. Extending upwardly from base portion 37a is a cylindrical chamber 37c, enclosing a resonator chamber 37d. As shown, the side walls of the chamber 370 are annularly spaced from the bore 36a, and the lower ends of the side walls are formed with a series of ports 37c, which ports constitute the mouth of the resonator. Enclosed within the chamber 370 is a steel ball 38. The resonator 37 functions in theusual manner to attenuate detonation frequency-waves of frequency to which it is resonant. The chamber being exposed to the flame, carbon accumulation therewithin is minimized. In addition, the steel ball 38 is free to rattle about inside the chamber owing to the reciprocating motion of the piston, and this rattling ball has a scouring action on the inside surfaces of the chamber, tending to prevent carbon accumulation. In passing, it may be noted that the attenuator 37 is one of several disclosed types which has been arranged in sound transmissive communication with the combustion chamber by being positioned virtually inside the chamber.

Figure 20 shows a resonant membrane absorber, in combination with a resonant absorber cavity. In this case, resonant absorber cavities 39, illustratively Helmholtz resonators, although pipe resonators would serve as well, are formed in combustion chamber wall 220, and are covered by relatively thin flexible or elastic plates 39a which are fastened to the combustion chamber wall by any suitable means such as welding. The detonation wave damping effect of the cavity is preserved, since the cavity and flexible plate combination present a sound wave reflecting means tuned to a predetermined resonant frequency matching the detonation frequency to be attenuated. The vibratory membrane or plate also absorbs energy from the sound wave, and by tuning it, in combination with any cavity which might be in back, to be resonant to the detonation frequency, its vibration amplitude becomes substantial, and a substantial amount of the acoustic energy may thus be absorbed, and the detonation wave reduced accordingly. This effect is of course over and above that owing to the cavity behind it. The plate 39a also serves the important function of preventing carbon deposit within the cavity. The attenuator of Figure 20 is illustrative of a type wherein a cavity is arranged in acoustic communication with the combustion chamber through a solid elastic member. The expression acoustic communication thus refers not only to transmission through a gas, but through any elastic media which will transmit the wave.

In-Figure 21 is shown a valve-in-head engine of modern type, having a water cooled block 40, water cooled head 41 fastened to block 40, and a piston 42 reciprocable in cylinder 43 within block 40. Head 41 comprises a generally domed or hemispherical combustion chamber wall 44 defining a combustion chamber 45 over piston 42. In the center of head wall 44 is a threaded port 46 to receive spark plug 47. Intake and exhaust valves open to combustion chamber 45 through wall 43, an exhaust valve being indicated at 48, and the intake valve (not shown) being understood to be located symmetrically with respect to the exhaust valve, forwardly of the plane of the drawing, as will be understood. Details of operating mechanism for these valves may beof a type known in the art and not requiring further illustration or description herein.

Formed in the generally hemispherical wall 44 of en- 7 gine head 41, somewhat below spark plug port 46, and

between the intake and exhaust valves, are threaded ports 49 and 50 to receive acoustic attenuators.

An illustrative acoustic attenuator 51, mounted in port 49, has a tubular housing 52 formed at one end with a reduced externallyscrewthreaded tubular stem. 53 which is screwed into port 49. The reduction in diameter to form stem 53 affords an upwardly facing internal annular shoulder 54 in body 52, on which is seated a porous body or wall 55. In the present illustrative embodiment, this member55 is in the physical form of a disklike wall or plug 55,. those external diameter is such as to be accommodated with a free sliding fit within the internal bore 56 of body 52. The upper or rearward end of bore 56 is screwthreaded to receive'screwheaded closure plug 58, and a resonator cavity 59 is formed within body 52 between the porous body 55 and said closure plug 58. Preferably, the space between body 55 and plug 58 is made suflicient to accommodate an inverted ceramic cup 60, which is mounted on body 55, and whose lower concave surface 61, together with the upper surface of the body 55, defines the resonator cavity 59.

Various materials have been found suitable for the purpose of the porous body 55. For example, I have found that fiber glass confined between screens is a very suitable material, having highly desirable acoustic absorber characteristics. I have also found that sintered powdered metal, such as used to form a common type of oil-impregnated bearings, is quite satisfactory. In making such a porous body, the powdered metal is first compressed under high pressure,*and then fired until the particles partially fuse and weld together. The degree of porosity can be varied by grain or particle size, and can thus readily be selected to give optimum performance in accordance with the teachings of the invention. I have also found porous ceramics, .as well as certain graphite granular compositions, to be quite satisfactory if used in sufficient area, Such a body or wall has tortuous tubes or passageways,- often of nearly capillarylike dimensions,.extending entirely or partially. through it between its opposite faces. These tubes or passageways may comprise inter-communicating pores, cells, crevices, or the type of intercommunicating air spaces formed in such a material as a body of fiber glass or the like, which may be 90% air space. The passageways, characteristically tortuous in nature, are open for fluid flow through the Wall or body, but are sufficiently constructed to cause substantial friction losses of a viscous ty e.

One such attenuator as thus described, connected to the combustion chamber, has a substantial attenuative effect on detonation, but an additional attenuator may also be connected into the head through the second port 50. This additional attenuator may be of the same type as the one already described, though I have here shown a modified type, having certain added features of advantage, as will be described in detail hereinafter.

As a further means for suppressing detonation sound waves within the combustion chamber of the engine of Figure 21, a porous pad, wall or body 67 is mounted on the top end of piston 42, and may also be of sintered, powdered metal. Such a pad may conventionally be silver soldered to the piston. Also, the top of the piston, beneath the pad 67, is shown to be formed with resonant absorber cavities 68,- here shown to consist of approximately quarter-wave cylindrical holes bored into the top of the piston and opening directly to the bottom surface 22 ers 68 are drilled to depths to be resonant to one or more detonation frequency bands as described in connection with the engine of Figure 9.

The tortuous passages in the porous wall or body in the attenuator 51 and'in the porous pad 67 mounted on the piston introduce substantial frictional resistance of a viscous type to the passage of sound waves The porous wall 55 incombination with the resonator cavity 59 has the ability toattenuatea wide'frequency range, as well as a narrow band of destructive detonation fre-' quencies to which'the resonator cavity is designed to be selectively resonant. The porous pad 67 mounted atop.

the piston further attenuates a wide frequency band; and the cavities 68 below the pad 67 give peaked response characteristics coinciding with one or more relatively narrowbands of destructive detonation frequencies.

Broadly considered, the main benefit of the porous Wall used across the neck of a resonant absorber is to materially reduce the Q of the resonator, flattening the peak of the response curve, and therefore increasing the frequency band width which will be attenuated. As earlier explained, the resonant absorber standing alone, such as either of the Helmholtz type or the pipe resonator type, has a relatively high Q, and while this Q factor is subject to some control in the design of the resonator, the attenuative response of such a device is somewhat restricted in band width. The use of the porous wall across the neck of such a resonator greatly increases the band width to which the attenuator will respond. The resonator in this case is designed to have its resonance peak at the detonation frequency which is to be attenuated, and by virtue of the broadening of the wave band resulting from use of the porous wall, not only that frequency, but other frequencies for a considerable range on either side thereof will be materially attenuated. This is of particular advantage where the total band width of a resonance peak is fairly large, and a sharp tuning device would not remove the entire peak. Also, the attenuator may sometimes be designed in this form to have aprincipal attenuation response to a predominant detonation frequency, but to be also responsive to a useful degree to some other detonation frequency. Thus such a broad tuning device as here described may have frequency response to two or more separated detonation frequencies. Of course, as already explained, the tortuouspassageways extending into it through the porous wall have an attenuative effect of their own, but this is not too important in this type of attenuator, since the attenuative effect of either the Helmholtz resonator or the pipe resonator is usually sufficient without such aid. The broadening of the response band is, however, often a useful feature.

The invention may also be practiced by using the porous wall for its ability to absorb sound waves even without the use of the resonator cavities behind them. For example, the porous pad 67 on top of the piston 42 may be used without 'the resonator cavities 68. The porous wall has a frequency response characteristic of its own, even without the resonant cavity behind it, but its Q (sharpness of tuning) is very low and the device in this form is responsive to a very wide frequency band. Nevertheless, the response characteristic of the attenuator in this form cannot be entirely disregarded, and it is desirable to make preliminary tests, according to known acoustical techniques, to assure that the pad used will have an attenuative response to the sound wave frequencies accompanying the detonation that occurs within the engine. The fine pores or passageways into and through the porous wall offer very great viscous friction to the sound waves which are incident thereon, and if the walls are made of suflici'ent thickness relative to the wavelength, virtually of the incident wave energy could be thus absorbed; In a practical engine installation, where the thickness of the porous wall is somewhat restricted, something less than 100% absorption will be gained, but 

